1. Field of the Invention
The present invention relates to a straddle carrier for transporting and stacking freight containers with automatic steering made up of two chassis beams having steerable wheels for traveling over the container stack.
The proposed straddle carrier is suitable for transshipment of containers, where there is a need to steer the vehicle automatically, in precise and reliable manner, along stacks of containers.
More specifically, the invention relates to a straddle carrier as used in ocean harbors and container terminals worldwide, for transporting and stacking freight containers. They consist of two chassis beams having wheels, which travel on the right and the left of the container stack, and vertical supports set onto them, which form a portal with the machinery platform that lies above them. This portal moves over the containers and picks them up with its lifting mechanism, or sets them down. The straddle carriers have a diesel-electric or diesel-hydraulic drive and, in most cases, an electro-hydraulic steering. A steering computer regulates the hydraulic proportional valves as determined by a steering wheel setting by the driver, which valves in turn move the wheels to the steering setting, by way of hydraulic cylinders.
Straddle carriers must be steered over long stacked container rows. The chassis beams travel on the right and the left of the stacks being traveled over, in the narrow alleys between the container rows. The vehicle operator must therefore steer very precisely and attentively, and also cannot drive very fast, because the clear space between the chassis beams and the containers is very narrow, and sideswipes of the vehicle against the containers and other collisions must be avoided. In order to support the driver and to relieve him of concentrated, exhausting steering, automatic steering would be desirable.
2. The Prior Art
Automatically steered straddle carriers are known. They use generally known methods, in most cases, as they are used for navigation for all autonomous, driverless lift trucks. Namely individual systems or combinations of:                satellite navigation (differential GPS),        radar navigation,        inertial navigation (gyroscopes),        transponder navigation (with markers buried in the floor),        guide wire navigationor the like are used. For fully automatic steering, these known navigation systems must allow determination of the vehicle position in the centimeter range, making them very complicated and costly.        
Navigation with laser scanners is also known. Thus, for example, DE 38 21 892 C1 shows determination of the position of a vehicle using laser scanners, whereby fixed reflectors must be installed on the light posts of the harbor facility, for example, and the laser scanner constantly measures distances and angles to the reflectors. The current vehicle position is determined by comparison with the known positions of these reflectors, which is stored in the memory of the electronic vehicle control. However, automatic steering of the vehicle using laser scanners is not indicated in DE 38 21 892 C1.
Furthermore, the use of laser scanners for different positioning and position determination tasks for cranes and container stackers is known, for example from DE 100 39 629 B4 and DE 44 15 419 A1, but not for fully automatic steering of a straddle carrier above the container row.
All of these known methods for fully automatic vehicle guidance have several disadvantages: They must be precise to the centimeter, and are therefore very complicated and orient themselves not by the container stack itself, but rather by external markers, such as satellites, reflectors, transponders, and the like. They are used to steer the vehicle along a path that is only theoretical, on which the container stack is supposed to stand. This path is stored in the memory of the vehicle control. If the containers are in fact not set down precisely on this theoretical path, contact, collisions, damage, and therefore interruptions in operation can occur. Furthermore, the paths must be changed in all the vehicle controls every time that the containers have to be stacked differently for logistical reasons. Such change is quite complicated to implement in terms of software technology.
Purely manual operation by the driver, on the other hand, has the disadvantage that the driver must steer the vehicle over the container stack, which is often several hundred meters long, very attentively and with great concentration. He or she tires, loses his or her ability to concentrate after some time, and must therefore drive relatively slowly, which limits the transshipment capacity of the vehicle. Also, he or she must constantly look vertically down onto the gap between chassis beam and container row for steering, and can observe the travel distance that lies ahead only with difficulty.
Furthermore, collisions frequently occur if the driver has not moved the lifting mechanism high enough, due to lack of attentiveness, when moving in over a container stack or when approaching a part of the container row that is stacked higher. Then the spreader or the load on the spreader hits against the container that is stacked higher, which causes significant damage.